1. Field of the Invention
The present invention relates to a method for producing a tail portion in production of a silicon single crystal by Czochralski method (CZ method), and a silicon single crystal produced thereby.
2. Description of the Related Art
In a conventional method for producing a silicon single crystal by CZ method, a silicon single crystal as a seed crystal is contacted with a silicon melt, and is slowly pulled with rotating to grow a silicon single crystal ingot. A heater is provided around a quartz crucible and a graphite crucible containing the melt to heat the crucible while the single crystal is pulled.
In order to eliminate a dislocation propagated from slip dislocation generated in the seed crystal in high density due to thermal shock after the seed crystal is contacted with the silicon melt, a neck portion is formed by decreasing a diameter of the crystal to about 3 mm, namely, so-called necking is conducted. The diameter of crystal is then increased to a predetermined value to form a cone part, and subsequently a straight body of the single crystal having the predetermined diameter (hereinafter referred to as a predetermined or constant diameter portion) at a predetermined rate, and thereby a silicon single crystal ingot is pulled.
When the single crystal is grown to have a predetermined length, a tail end (a tail portion) of the single crystal is separated from the silicon melt. If the grown single crystal is simply separated from the melt, temperature at a separated portion of the single crystal rapidly decreases, and slip dislocation is generated in the single crystal, which lowers a rate of crystallization (yield of a predetermined diameter part of single crystal having no problem in quality).
To solve the above problem, the diameter of the crystal is gradually decreased after the predetermined diameter part of the single crystal is produced so that a contact area of the single crystal with the melt can be sufficiently decreased, and then the single crystal is separated from the melt. Generation of slip dislocation can be thus prevented. Generally, the part where the diameter is gradually decreased is called "a tail" or "a rounded part".
The tail part is conventionally formed by growing crystal with gradually increasing temperature of a melt by increasing quantity of heat supplied from a heater to a melt in a crucible, and with gradually decreasing a diameter of the crystal. However, in the method, the temperature of the melt after the tailing is so high that defects called OSF (Oxidation Induced Stacking Fault) is apt to generate in ring-like distribution (hereinafter may be referred to as OSF ring). Furthermore, temperature is changed much when the single crystal is separated from the melt, so that the single crystal is rapidly cooled and there is formed a part where an amount of precipitated oxygen is extraordinarily large (hereinafter occasionally referred to as extraordinary oxygen precipitation area).
In the tailing process, the pulling rate of single crystal is sometimes increased. However, it may result in separation of the rounded part halfway. Furthermore, since the pulling rate is rapidly increased to conduct the rounding process after the predetermined diameter part is grown, temperature changes rapidly, and the single crystal is thus cooled rapidly, which may lead to generation of slip dislocation, so that the extraordinary oxygen precipitation area may generate even in the predetermined diameter part.